Question

$QUESTION \: :$
Why only HBr undergoes Kharsch effect with peroxide?​

Answer 1

★ ǫᴜᴇsᴛɪᴏɴ

Why only HBr undergoes Kharsch effect with peroxide !?

★ ᴀɴsᴡᴇʀ

In the presence of peroxide and light, addition of HBr to unsymmetrical alkenes occur contrary to Markovnikov's rule .

But HCl and HI do not show peroxide effect . Radical chain reactions are successful when propagation steps are exothermic. An endothermic propagation step is reversible and would break the chain. With HI, the first propagation step is endothermic, because H-I bond is weak. Moreover, HI is oxidized by peroxide to I2 under reaction conditions.

This is the most probable reason .

ʜᴏᴘᴇ ɪᴛ ʜᴇʟᴘs

Answer 2

Answer:

${\huge{\green{ANSWER}}}$

HCl is a very stable acid H-Cl bond (430 kJ moH) is stronger than H-Br bond (378 kJ mol-1) and is not broken symmetrically by the free radicals generated by peroxide. Hence the free radical addition of HCl to alkenes is not possible.